Electromagnetic locks for securing doors or gates are well known in the prior art. In a typical installation, a magnetically-susceptible keeper plate is mounted on a door, and an electromagnet is mounted on a door frame. When the electromagnet is energized and is in contact with the keeper plate with the door closed, the plate becomes an armature for the electromagnet, thus providing a mechanism for locking the door to the frame. When the magnetic loop is complete, by contact of the armature with the electromagnet, the magnetic flux density is at a maximum.
In some access control systems used, for example in doors used in conjunction with fire control and emergency exits, it is desirable to provide delayed egress through an emergency door. Delayed egress is when an access control system must provide guaranteed egress within a fixed period of time while also providing notification to security personnel during that same period of time that egress is required. During the delayed egress time period, while a small gap exists between the door and frame, the door is kept in the locked state with power continuing to be supplied to the electromagnet. At the end of the delay period, power is removed from the electromagnet thus allowing free egress. Within the time period before de-energizing, a shorter “nuisance delay” period exists to deal not only with accidental striking of the door, but also with thwarted attempts of vandalism as might be expected from young persons who would push the door, hear the alarm, and then run away. If action to open the door ceases during the nuisance delay period, the alarm also ceases and the system remains armed for the next opening attempt. On the other hand, if the attempt to open the door is sustained for longer than the nuisance delay period, say after 15 or 30 seconds, the signal to open the door becomes irrevocable, the electromagnet is de-energized at the end of the irrevocable time, and the door is permitted to fully open. Such systems are in broad use, particularly in retail establishments where they greatly reduce theft loss while complying with building codes that require a minimum number of emergency exits.
In known systems, a gap can occur between the frame and the door because the keeper plate (armature) is not rigidly fastened to the door but rather floats on an armature mounting device which is fixed to the door. See, for example, U.S. Pat. No. 6,609,738 B1, the relevant disclosure of which is incorporated herein by reference. Consequently, pressure on the emergency door can create a gap between the door and the door frame of, typically, up to about one inch (2.54 cm.). In some security applications, it is essential that the door not be allowed to clear the frame during the nuisance delay period, to prevent passing of, for example, documents; thus, in some applications a gap as small as about 0.125 inches (0.32 cm.) must be detected.
It is known in the cited prior art to employ a digital device based upon the Hall Effect, wherein the voltage potential orthogonal to a magnetic field is proportional to the strength of the magnetic field. Thus, attempted opening of an emergency door increases a starting gap between the sensor and a cooperating permanent magnet mounted on the door, reducing the strength of the sensor magnetic field, which reduction can be sensed by the Hall Effect sensor and an alarm provided by a cooperating micro-controller.
A known problem in use of prior art systems, such as is disclosed in U.S. Pat. No. 6,609,738, is that the digital Hall sensor signals can alarm only when the sensor/magnet gap reaches a predetermined value. Thus, the door/frame gap for each installation requires individual physical adjustment to obtain proper correlation between a desired door/frame gap setting and the preset sensor/magnet gap. For this reason, employing a single system design to accommodate a range of desired door/frame gap settings is unwieldy.
What is needed in the art is an improved Hall Effect gap detection and alarm system that may be easily set to provide an alarm at any desired gap size for any installation.
It is a principal object of the present invention to simplify the setting of an alarm or nuisance gap at any desired gap size for an electromagnetic lock installation, thus making an alarm system applicable to a wide range of door/gap requirements.